In order to increase performance and operational reliability of transformers, efficient and reliable cooling of transformer components can be desirable. Active cooling systems which use oil, for example, as a cooling medium are known from power transformers which operate in a voltage range, for example, up to 800 kV. Large power transformers can be installed in systems in which a high short-circuit power can occur. The mechanical stress which can occur in short-circuit events can influence the design. Modern power transformers can use oil both for electrical insulation and for cooling. The oil can be actively pumped through the transformer, and heat is extracted from the circuit by heat exchangers.
Distribution transformers can be in the form of oil-filled models, or dry-insulation models. Oil-filled distribution transformers can have primary voltages up to 72.5 kV, while dry transformers, with open or encapsulated turns, can have primary voltages up to 52 kV and can be used where the risk of fire or environmental contamination should be minimized.
The use of fans and transverse air blowers is known for cooling dry transformers. With this type of active forced cooling, air can be blown through channels which are located in the transformer. The channels which are used to guide the air may be, for physical reasons, located only between the windings on a primary side of the transformer coil. Because the turns on a secondary side may be typically wound first in a spiral shape in a radial direction in a plurality of layers and then the next axially adjacent turn is wound in the same wound manner, it can become difficult to provide cooling channels. The secondary side of the dry transformer, which can be encapsulated with epoxy resin, can therefore provide cooling via thermal conduction and convection on the surface of the epoxy resin. The performance of many drive transformers which are cooled in this way can be limited by the operating temperature on the secondary side of the transformer. Active cooling systems may not operate autonomously, and can fail more easily.
Another type of cooling for dry transformers is disclosed in U.S. Pat. No. 5,656,984. There, passive cooling is disclosed, in which four heat pipes are placed close to a transformer core, and primary turns are wound around them. The evaporators for the individual heat pipes are, for this purpose, in the form of thin pipes which run in an elongated form, and run parallel to the winding axis. As a result of the very local and singular arrangement of the heat pipes, for example, in the radial direction of the transformer, the heat which is created and results, for example, from transmission losses and hysteresis, can be extracted in the areas close to the individual heat pipes. The cooling power can therefore to an extent vary locally, and depends on the arrangement of the individual heat pipes in the transformer. This design involves a plurality of heat pipes which, for example, can cool zones close to the transformer core.
U.S. Pat. No. 4,129,845 proposes active transformer cooling in which the transformer to be cooled is sheathed by a container in which a liquid reservoir is also located. A pump conveys the cooling liquid from the reservoir, which is located underneath the transformer, upward, and distributes it above the transformer windings. The cooling liquid which has been distributed in this way flows down between the turns of the transformer, and evaporates.
U.S. Pat. No. 6,368,530 discloses a cylindrical, coolable transformable transformer coil in which four cooling channels are provided between the inner and the outer turns. The cooling channels run in the direction of the rotation axis, and are separated from one another by equal distances.
GB 764576 discloses a coolable transformer which is surrounded by a container which is filled with cooling liquid. The evaporating cooling liquid can rise into the lines, which act as a condenser, can condense there and can flow back.
JP 6215959 discloses a use of heat pipes for passive cooling of transformers. There, a multiplicity of evaporators in the form of rods are placed concentrically and at an axial distance from one another around the transformer core, or run bent in a U-shape in the axial direction between the individual turn layers.
An improved cooling concept to achieve a higher power density in dry transformers is desirable.